The present invention relates generally to apron drafting systems for textile fibers, and more particularly to an apron arrangement for use in a three or more roll system of the type used in spinning and roving frames.
Most spinning frames, as well as roving frames, today use what is known as a "three-roll system" in which there are three pairs or sets of upper and lower rolls at spaced intervals, the textile fibers being fed initially to the back rolls from which they pass to the middle rolls and then to the front rolls from which the drafted fibers pass through a twisting arrangement to a spindle-mounted bobbin on which the finished yarn or roving is wound. In each of the pairs of rolls, the bottom roll is driven at a predetermined speed, with the back bottom roll rotating at the lowest speed and the intermediate bottom roll being driven at a slightly higher speed. The front bottom roll is driven at a still higher speed, and the ratio of the speeds constitutes the draft ratio. That is, the speed ratio between the middle bottom roll and the back bottom roll defines the break or back draft ratio, and the speed ratio between the front bottom roll and the middle bottom roll is the final or front draft ratio. Generally, the back draft ratio is relatively small and, conventionally, is between 1.2:1. and 1.5:1. The front draft ratio is much higher, and may range between 15:1 and 30:1. Since the total draft ratio is the product of the two ratios, the total draft ratio is therefore generally between 18:1 and 45:1 for spinning frames. While the higher ratios are desired to provide flexibility to spin yarn counts from coarser roving, such higher ratios tend to produce yarns of lower quality because they introduce unevenness in the cross-sectional density of the yarn. The magnitude of unevenness is related to the amount of draft.
The top rolls are freely rotatable, being driven by the adjacent bottom roll, and a top arm arrangement is used to support and position the top rolls and apply a spring-biasing or weighting force on the top rolls to provide the proper weighting or compressive force between the respective top and bottom rolls. Furthermore, the spacing between the sets of rolls is generally adjustable, and can be varied depending upon the staple length of the particular textile fibers.
In order to provide control for the fibers in the front drafting zone, to provide better evenness of the fibers in view of the high draft ratios in this zone, an apron guiding system is provided utilizing flat rubber aprons extending both above and below the path of the fibers. Conventionally, the bottom apron passes around the bottom middle roll forwardly toward the front bottom roll, where it passes over a guide or apron pin and returns to the middle bottom roll directly below the fiber path. Likewise, the top apron extends around the top intermediate roll forwardly in close abutting proximity with the bottom apron roll around an upper deflecting or apron pin and back above the fiber path to the top middle roll. Generally, the guide pins or apron pins are arranged as close as possible to the nip between the front rolls, so that in this front drafting zone the fibers can be guided between the two aprons for the maximum possible distance in the drafting zone. The structures for the mounting of the aprons fall generally into two types. One of these is the so-called "split cradle" design, in which the structures for mounting and guiding the two aprons are separate, and when the top arm is raised to raise the top rolls, the top apron and its mounting assembly move with it to open up and expose the complete fiber path between the back and front rolls. The other type is the one-piece cradle, in which the top and bottom aprons and the guide pins are mounted on a single cradle member which is positioned by both the top and bottom middle rolls in such a way as to be securely held in place when the top arm is in the clamped position.
Although the above-described apron arrangement has received wide usage, various other apron arrangements have been proposed, including extending the same aprons into the back drafting zone around suitable deflecting elements in these zones.
One such arrangement is shown in German patent No. 938,898 utilizing only a bottom apron without a top apron, or with top and bottom aprons, and utilizing rollers in the back drafting zone adjacent the back rolls so that each of the aprons extends in abutting contact with the other apron from a pair of rollers in the back zone close to the back rolls through the nip between the middle rolls to a conventional apron guide pin adjacent the front drafting rolls.
Another arrangement similar to that of the above patent has been shown in U.S. Pat. No. 4,067,088, which shows a split cradle arrangement having apron guide bars for both the top and bottom aprons in both the front and back draft zones, and showing an arrangement in which the aprons extend a greater distance in the back draft zone than in the front zone.